dddhgg
sage
Reged: 08/03/07
Posts: 265
Loc: The Netherlands
|
|
I don't know much optical theory, so please don't shoot me for asking this. :-) Why can't the secondary mirror in a classical cassegrain of long focal ratio simply be made flat instead of hyperbolic?
Many thanks in advance!
--------------------
Meade 8" SCT (an older white-tubed version)
Celestron CG5 (non-GOTO)
Teleskop-service 30 mm WA
Teleskop-service 7-21 mm zoom
GSO 2" standard star diagonal
|
BluewaterObserva
Post Laureate
Reged: 05/18/04
Posts: 4763
Loc: Zuni Mtns, NM
|
|
It has to extend the focal length to get past the primary, so the eye piece comes out the back / end of the scope.
|
dothead
super member
Reged: 08/13/08
Posts: 138
Loc: Heidelberg, Germany
|
|
Because in order to put the focal point behind the rear end of the OTA, you'd need a flat with a diameter of more than 50% of the primary (this system would be called a folded Newtonian (not a Cassegrain)). Impossible for visual use due to extremely diminished contast (more exactly: MTF-curve); even for phtographical application, a 50%+ obstruction would be problematical.
There are folded Newtonians, but their secondary flat is sized smaller than 50%, so the focal point would lie inbetween the secondary and the primary. To get access to the focal point, a tertiary flat is then used to redirect the beam towards the side of the tube (like the secondary flat in an ordinary Newton does).
Regards,
Ralph
|
BluewaterObserva
Post Laureate
Reged: 05/18/04
Posts: 4763
Loc: Zuni Mtns, NM
|
|
Interesting. I would of moved it to cats. 
|
dothead
super member
Reged: 08/13/08
Posts: 138
Loc: Heidelberg, Germany
|
|
Question to the mods:
why doesn't a Classical Cassegrain belong to the Reflector forum?
It surely is NO CATiadoptric (no refracting lenses here)!!!
And why now in the ATM forum?
I note that in the description of the Reflector forum, it says: "A forum to discuss all types of reflecting telescopes, Newtonian, Cassegranian(Classical), Herschelian, Off-Axis, etc..."
The irony of moving this thread away from the Reflectors forum is the fact that the original poster envisioned/proposed a folded NEWTONIAN, albeit unintentionally!
I'd appreciate if the Reflector forum mods would discuss this issue.
Sorry for the rant!
Ralph
|
jgraham
Postmaster
Reged: 12/02/04
Posts: 5386
Loc: Dayton, Ohio
|
|
The orignal goal of the calssical cass design is to put a long focal length telescope into a small tube. The convex secondary serves the same function as a barlow, but without the chromic aberation problems.
There is a design for a 'cass' with a flat secondary, but it's not called a cass, but a folded Newtonian instead. Therer are no real benefits of the design other than a shortened tube and the one example I'm aware of was designed as a travel scope. The secondary was so huge (50% CO) and cast such a huge shadow on the primary the builder used an un-perforated primary; instead a tertiary mirror was placed in front of the primary under the shadow of the secondary.
--------------------
-John
================================================
Homebuilt scopes from 4.25-16.5"
Meade LXD75-N6/SN6/SC8, DSX-90, ETX-60BB, ETX-125PE, DS-2130
Orion StarBlast, BinoViewers, Coronado PST
Rebel XT/XTi, DSI Pro (I & II), DSI, LPI, Electronic Eyepiece, Phillips SPC900NC
Tasco 60mm Refractors
|
Don W
Postmaster
Reged: 05/19/03
Posts: 13244
Loc: Wisconsin, USA
|
|
This is not just the ATM forum. It is also the Optics forum. The Reflectors forum is meant mainly for Newtonians.
--------------------
Don Wyman
Obsession 18" f/4.5 #1166
W/Argo Navis DSC and Torus Primary
William Optics Megrez 90
Coronado PST
|
dothead
super member
Reged: 08/13/08
Posts: 138
Loc: Heidelberg, Germany
|
|
John,
of course you could also build a Classical Cassegrain (parabolical primary, hyperbolical secondary), use an unperforated primary and place a tertiary flat in the shadow of the secondary to redirect the beam to the side of the tube (preferably through a bore in the declination axis to keep the eyepiece stationary) - this is then called a Nasmyth (-Cassegrain). This layout is not uncommon with big observatory reflectors.
I wonder why no commercial manufacturer has tried to implement this for big alt-az-mounted amateur reflectors (such as 25+ inch dobs). This would allow to observe in a comfortable seated position instead of balancing on a tall ladder.
To see a 32inch custom Nasmyth telescope Philipp Keller has built, go to www.astrooptik.com, then click "Projekte/Kunden" on the left side, the again "Pollux" on the left side. The scope can be either used as a F/4 Newtonian or a F/15 Nasmyth.
Regards,
Ralph
|
dddhgg
sage
Reged: 08/03/07
Posts: 265
Loc: The Netherlands
|
|
Quote:
Because in order to put the focal point behind the rear end of the OTA, you'd need a flat with a diameter of more than 50% of the primary (this system would be called a folded Newtonian (not a Cassegrain)). Impossible for visual use due to extremely diminished contast (more exactly: MTF-curve); even for phtographical application, a 50%+ obstruction would be problematical.
There are folded Newtonians, but their secondary flat is sized smaller than 50%, so the focal point would lie inbetween the secondary and the primary. To get access to the focal point, a tertiary flat is then used to redirect the beam towards the side of the tube (like the secondary flat in an ordinary Newton does).
Regards,
Ralph
Thanks! :-) After drawing some diagrams, I think I understand why. But couldn't the focal point of my arrangement just be left before the rear end of the telescope, after which the image is led out of the tube with a couple of barlow-like lenses? Perhaps you could even use the baffle tube to attach these lenses to.
--------------------
Meade 8" SCT (an older white-tubed version)
Celestron CG5 (non-GOTO)
Teleskop-service 30 mm WA
Teleskop-service 7-21 mm zoom
GSO 2" standard star diagonal
Edited by dddhgg (08/16/08 09:44 AM)
|
dothead
super member
Reged: 08/13/08
Posts: 138
Loc: Heidelberg, Germany
|
|
Yes, one could do it that way, but still, the secondary flat would not be significantly less than 50% (don't forget that you also need some backfocus) - and because of the barlow, you'd have a slow scope.
The same f-ratio as above will be achieved by a Classical Cassegrain (hyperbolic secondary), with significantly smaller central obstruction than your proposed barlowed folded Newtonian.
Ralph
|
deSitter
scholastic sledgehammer
Reged: 12/09/04
Posts: 782
|
|
The Schmidt-Newtonian is essentially a Schmidt-Cassegrain with a flat secondary (spherical in general). Similarly a straight Newtonian is in a sense a classical Cassegrain with a flat secondary (hyperbolic in general). (A plane can be thought of as a sphere or a hyperboloid of infinite radius.)
-drl
|
dddhgg
sage
Reged: 08/03/07
Posts: 265
Loc: The Netherlands
|
|
By the way, looking on the web, I've found out that there seems to be at least one major research telescope with my proposed system (without the barlow though): the 1.55 m modified cassegrain telescope owned by the United States Naval Observatory at their Flagstaff facility. See web link http://ftp.nofs.navy.mil/about_NOFS/telescopes/ksar.html.
The central obstruction seems to be a bit more than 50%.
--------------------
Meade 8" SCT (an older white-tubed version)
Celestron CG5 (non-GOTO)
Teleskop-service 30 mm WA
Teleskop-service 7-21 mm zoom
GSO 2" standard star diagonal
|
Larry Geary
sage
Reged: 09/24/06
Posts: 472
Loc: NJ, USA
|
|
Quote:
Thanks! :-) After drawing some diagrams, I think I understand why. But couldn't the focal point of my arrangement just be left before the rear end of the telescope, after which the image is led out of the tube with a couple of barlow-like lenses? Perhaps you could even use the baffle tube to attach these lenses to.
I believe this is called a relay scope, and something like it was discussed (or maybe exhibited) at this year's Stellafane.
--------------------
1-20-2013
|
Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 1963
Loc: Boechout, Belgium
|
|
Did not read the whole thread, so sorry if I post something that has been posted before...
Obstruction in a Folded Newt is somewhere between 30 and 35%.
--------------------
Chris
|
dothead
super member
Reged: 08/13/08
Posts: 138
Loc: Heidelberg, Germany
|
|
Yes, but not in dddhgg's proposed configuration!
|
refractory
Pooh-Bah
Reged: 02/05/05
Posts: 1016
|
|
Ralph has me thinking one could include in a Nasmyth design a big beam-splitter- that way TWO people could enjoy the same view while sitting on opposite sides of large OTA, sort of the way teaching microscopes are set up. I know there were some old-fashioned focuser-end setups for this purpose (I've seen them for sale) that never really caught on- heads too close together for comfort?
Anyway, if the chairs were on some sort of giant carousel sitting atop the azimuth mechanism, then you'd be set- anyone make lazy suzans that big?
Jess Tauber
|
Previous
Index
Next
Threaded
Edit
Reply
Quote
[Re: 